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https://issues.apache.org/jira/browse/AVRO-160?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12769302#action_12769302
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Doug Cutting commented on AVRO-160:
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> It is useful to leave open the option for index type metadata in the metadata
> block.
I don't see the use case. Blocks should be small enough that seek time
dominates, so scanning should not be a dominant cost. Note that scanning is
required anyway when blocks are compressed. Avro scanning should be at least
as fast as decompression.
> if we ever want true optimized random access [ ... ]
So long as we're supporting compression, we'll never support seeks directly to
individual entries. But blocks are a relatively constant size, so we can
support constant-time access to individual entries.
> This allows MapReduce to not have to "seek and scan" but instead find the
> start of the metadata block nearest the HDFS block boundary.
Yes, if we kept a global block index, we could avoid this scan. However since
HDFS blocks are ~64MB, and Avro file blocks are ~64k, the scan is less than a
tenth of a percent of the overall map cost, so this is perhaps not a worthwhile
optimization.
> Maybe, a straightforward thing to do is consider that each block in this file
> has a header, a data block, and a footer.
That could work. We'd also need to terminate blocks with the length of their
footer metadata, so that a reader can efficiently find the last footer on open,
where, by convention, global data is written.
> file format should be friendly to streaming
> -------------------------------------------
>
> Key: AVRO-160
> URL: https://issues.apache.org/jira/browse/AVRO-160
> Project: Avro
> Issue Type: Improvement
> Components: spec
> Reporter: Doug Cutting
>
> It should be possible to stream through an Avro data file without seeking to
> the end.
> Currently the interpretation is that schemas written to the file apply to all
> entries before them. If this were changed so that they instead apply to all
> entries that follow, and the initial schema is written at the start of the
> file, then streaming could be supported.
> Note that the only change permitted to a schema as a file is written is to,
> if it is a union, to add new branches at the end of that union. If it is not
> a union, no changes may be made. So it is still the case that the final
> schema in a file can read every entry in the file and thus may be used to
> randomly access the file.
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